Isolated stripped segments of small intestine from Amphiuma will be incubated in a chloride-based buffer containing 25 mM HCO3 negative. Either of three protocols will be conducted. The intracellular chloride activity of villus absorptive cells will be measured in some segments using double-barreled chloride selective microelectrodes, one barrel containing Corning chloride exchanger and sensing chloride activity, the other measuring membrane potential. Additional segments will be incubated in H3-mannitol to estimate extracellular water then digested in nitric acid for measurement of total tissue chloride. This will allow an estimate of intracellular chloride concentration. Finally, other segments will be clamped between halves of lucite chambers and short-circuited. Unidirectional mucosa to serosa and serosa to mucosa fluxes of sodium and chloride will be determined on adjacent segments using the isotopes Na22 and Cl36. The net fluxes of these ions will be compared with the short-circuit current thereby allowing determination of any residual flux which may be contributing to the current generated by the epithelium. The influence of theophylline, acetazolamide, removal of HCO3 negative, Cl negative or Na positive will be assessed. The data will be examined to elucidate the chloride transport properties of the small intestine during absorption and following electrolyte secretion induced by theophylline.